Physics

Model (science)

A representation that explains and predicts the behaviour of real objects or systems, e.g. describing sound and light as waves.

Wave

A disturbance that carries energy from one place to another without transferring matter. The particles only oscillate around a fixed point.

What happens to particles in a wave

They oscillate (move back and forth) around a fixed position and pass energy on; they don't travel with the wave. A cork bobbing on water shows this.

Medium

The substance a wave travels through, e.g. air, water or a solid.

Equilibrium position

The rest position of the particles when there is no wave.

Crest (peak)

The highest point of a wave. In a longitudinal wave the equivalent is a compression.

Trough

The lowest point of a wave. In a longitudinal wave the equivalent is a rarefaction.

Wavelength

The distance from one crest to the next crest (or trough to trough). It is a distance, measured in metres.

Amplitude

The maximum displacement from the equilibrium position. It relates to the energy of the wave (loudness for sound, brightness for light).

Frequency

The number of waves that pass a point each second. Its unit is the hertz (Hz).

Period

The time taken for one full wave to pass a point, measured in seconds.

Wave speed equation

v = f x wavelength. Speed (m/s) equals frequency (Hz) multiplied by wavelength (m).

Frequency and period

f = 1/T and T = 1/f. They are reciprocals.

Frequency vs wavelength at constant speed

Inversely proportional: a higher frequency means a shorter wavelength.

Steps for wave calculations

Write down the data and convert units; write and rearrange the equation; substitute; give the answer with units.

Unit conversions for waves

cm / 100 = m; nm x 10^-9 = m; kHz x 10^3 = Hz; MHz x 10^6 = Hz; GHz x 10^9 = Hz.

Wave speed example (2 Hz, 3 cm wavelength)

Convert 3 cm to 0.03 m, then v = 2 x 0.03 = 0.06 m/s.

Transverse wave

A wave in which particles vibrate at right angles (perpendicular) to the direction the wave travels. Examples: light, waves on a rope, a Mexican wave.

Longitudinal (compression) wave

A wave in which particles vibrate back and forth parallel to the direction the wave travels. Examples: sound and a slinky push-pull.

Compression

A region in a longitudinal wave where particles are bunched together (high pressure).

Rarefaction

A region in a longitudinal wave where particles are spread apart (low pressure).

Surface water wave type

Both longitudinal and transverse, because particles move in circles.

Is a heat wave a wave?

No. Particles are not oscillating back and forth, so it is not a wave in the scientific sense.

Mechanical wave

A wave that needs a medium (particles) to travel through, so it can't travel in a vacuum. Examples: sound, water waves.

Non-mechanical wave

A wave that does not need a medium and can travel through a vacuum. Examples: all electromagnetic waves, including light.

Sound: wave type

Longitudinal and mechanical.

Light: wave type

Transverse, non-mechanical and electromagnetic.

Why astronauts can't hear on the Moon

There is no air, so sound (a mechanical wave) can't travel. Light and radio waves (EM waves) can travel through a vacuum.

Pitch

How high or low a sound is. It depends on frequency: higher frequency means shorter wavelength and higher pitch.

Volume (loudness)

How loud a sound is. It depends on the amplitude: a larger amplitude means a louder sound.

Speed of sound in different materials

About 346 m/s in air, 1481 m/s in water and 5120 m/s in iron.

Why sound is faster in solids

The particles are closer together, so vibrations pass between them more quickly.

Echo

A reflection of a sound wave from a surface.

Echolocation

Bats send out sound waves and listen for the echoes to find prey and objects.

Light wave structure

Oscillating electric and magnetic fields, at right angles to each other and to the direction of travel, so light is transverse and needs no medium.

Speed of light (EM waves in a vacuum)

3.0 x 10^8 m/s (300 000 km/s). All EM waves travel at this speed in a vacuum.

How radio waves are produced

Electrons are accelerated up and down in an antenna.

How visible light is produced

Electrons in atoms move between electron shells and release energy as light.

EM spectrum order (low to high frequency)

Radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.

EM frequency, wavelength and energy

As you go from radio to gamma, frequency and energy increase and wavelength decreases.

Radio waves

Longest wavelength, lowest frequency and energy. Used for broadcasting and communication.

Microwaves

Used for cooking (microwave ovens), mobile phones and communication.

Infrared

Heat radiation. Used in remote controls and night-vision cameras; warm objects give it off.

Why a night-vision camera sees people

Warm bodies give off infrared radiation, which the camera detects.

Visible light colours in order

Red, orange, yellow, green, blue, indigo, violet (lowest to highest frequency).

Ultraviolet

Higher energy than visible light. Used for sterilising and detecting forgeries; can cause sunburn and skin cancer, and the ozone layer blocks most of it.

X-rays

Used for medical imaging because they pass through soft tissue but not bone. They are harmful.

Gamma rays

Highest frequency and energy, and the most penetrating. Come from radioactive substances; used for cancer treatment and sterilising; harmful.

EM waves harmful even at low intensity

Gamma rays, X-rays and ultraviolet.

Why astronomers use all EM waves

All EM waves travel through the vacuum of space, so they can carry information from distant objects.

Why an object has a colour

It reflects that colour of light and absorbs the others.

White vs black surfaces

White reflects all colours; black absorbs all colours, so black surfaces heat up more in sunlight.

Colour of a yellow object in blue light

Dark or black, because there is no yellow light to reflect and the blue is absorbed.

What can happen when a wave hits matter

It can be reflected (bounces off), absorbed (energy taken in) or transmitted (passes through, possibly refracted).

Absorption of a wave

The wave's energy is taken in by the particles, which vibrate faster, so the energy becomes heat.

Normal (ray diagrams)

An imaginary line at 90 degrees to the surface at the point where the ray hits. Angles are measured from it.

Law of reflection

The angle of incidence equals the angle of reflection, both measured from the normal.

Angle of incidence

The angle between the incoming ray and the normal.

Angle of reflection

The angle between the reflected ray and the normal.

Regular vs diffuse reflection

Regular: a smooth surface reflects parallel rays in the same direction, giving a clear image. Diffuse: a rough surface scatters light in many directions, so no clear image forms.

Plane (flat) mirror

Reflects regularly. The image is the same size and the same distance behind the mirror as the object is in front.

Concave mirror

A mirror that curves inward and converges light, used in torches, headlights and satellite dishes.

Convex mirror

A mirror that curves outward and spreads light, giving a wide view. Used in car side mirrors and shop security mirrors.

Refraction

The change in a wave's speed (and wavelength) when it enters a new medium; if it hits at an angle it also changes direction. Frequency stays the same.

Light from air into glass or water

It slows down and bends towards the normal.

Light from glass or water into air

It speeds up and bends away from the normal.

Light hitting glass at an angle

Some light is reflected, some is refracted into the glass, and some may be absorbed.

Convex lens

Thicker in the middle. It converges (brings together) light to a focal point and can project an image. Used in magnifying glasses, cameras and telescopes.

Concave lens

Thinner in the middle. It spreads light out and can't project an image. Used in glasses for short-sightedness.

The eye as an optical device

The convex lens focuses light onto the retina; photoreceptors send signals along the optic nerve to the brain.

Static electricity

Electrons are transferred from one material to another. The material that gains electrons becomes negative; the one that loses them becomes positive.

Like and unlike charges

Like charges repel each other; opposite charges attract.

Conductor vs insulator

A conductor lets electrons flow easily (metals). An insulator doesn't (plastic, rubber).

Electric circuit

An unbroken loop containing an energy source, conductors (wires) and a load (e.g. a globe).

Current

The flow of electric charge (electrons). Symbol I, unit ampere (A), measured with an ammeter connected in series.

Voltage

The 'push' that drives current around a circuit. Symbol V, unit volt (V), measured with a voltmeter connected in parallel.

Resistance

How much a component opposes the flow of current. Symbol R, unit ohm.

Ohm's law

V = I x R, so I = V / R and R = V / I. More resistance means less current for the same voltage.

Gradient of a V-I graph

The gradient (V divided by I) equals the resistance.

Series circuit

Components are on one path. The current is the same everywhere, and if one bulb fails they all go out.

Parallel circuit

Components are on separate branches. Each branch gets the full voltage, and if one bulb fails the others keep working.

Advantages of parallel circuits at home

Each appliance gets full voltage, can be switched on and off independently, and the others keep working if one fails.

Electrical power

The rate at which electrical energy is converted. P = V x I, measured in watts (W).

Electrical energy

E = P x t = V x I x t, in joules when time is in seconds.

Kilowatt-hours

Energy in kWh = power in kW x time in hours. 1 kW = 1000 W.

Current in a 60 W bulb on 240 V

I = P / V = 60 / 240 = 0.25 A, so R = V / I = 960 ohms.

Kettle: 24 ohm on 240 V

I = V / R = 10 A, and P = V x I = 2400 W.

Law of conservation of energy

Energy cannot be created or destroyed, only transferred from one object to another or transformed from one form to another. The total stays the same.

Energy transfer vs transformation

Transfer: energy moves from one object to another (a hot cup warms your hands). Transformation: energy changes form (chemical to kinetic in muscles).

Energy conversion: electric stove

Electrical energy to heat.

Energy conversion: loudspeaker

Electrical energy to sound.

Energy conversion: power drill or lawn mower

Electrical energy to kinetic energy (plus some heat and sound).

Energy conversion: TV

Electrical energy to light and sound (plus some heat).

Energy conversion: solar cell

Light energy to electrical energy.

Efficiency

The percentage of the input energy that becomes useful output: efficiency = useful output / total input x 100%.

Inefficient appliance

One that wastes a large share of its input energy, usually as heat and sound spread into the surroundings. The energy isn't destroyed, just not useful.

Kinetic energy

The energy of a moving object. Ek = 1/2 x m x v squared. (Check whether it is on your formulae sheet.)

Gravitational potential energy

Energy stored because of height above the ground. Ep = m x g x h. (Check whether it is on your formulae sheet.)

Conduction

Heat transfer by vibrating particles passing energy to neighbouring particles, mainly in solids. Metals are good conductors.

Convection

Heat transfer in liquids and gases: hot fluid expands, becomes less dense and rises while cooler fluid sinks, forming a convection current.